Factors that Affect Stopping Distances on the Road

Safety - 30 April 2019

It is the general belief that a car’s brakes are what will enable you to stop suddenly to avoid a crash. Not so, and we’ll explain why.

How do Brakes Work?

Brakes are the system responsible for turning the kinetic energy of your moving car into heat (science talk for slowing it down), but the car’s tyres are ultimately responsible for how quickly this happens.

Most modern passenger cars are equipped with electronic safety aids to maximise braking potential, the most important of which is the anti-lock braking system (ABS), which prevents your wheels from locking under hard braking and allows you to retain control over the steering in an emergency. When ABS is activated, it tells the onboard systems that your wheel has stopped rotating and that you have exceeded the maximum stopping force of the tyre. Your brake may be capable of applying more pressure, but the tyre has lost grip and more pressure serves no purpose. In other words, your stopping distance is limited by the tyre.

Electronic brake force distribution is another safety feature that ensures that braking power is dynamically assigned to each wheel in varying amounts according to the grip of the surface.

What is Stopping Distance?

The stopping distance, or braking distance, is the distance a car travels before it stops completely after you have applied the brakes fully. It is typically given as a 100 km – 0 km figure.

How is Stopping Distance Calculated?

Braking distance is a mathematical calculation of:

original speed of the vehicle

the type of brake system in use

the reaction time of the driver/rider

the coefficient of friction between the tyres and the road surface - also known as the traction coefficient.

Experts agree that traction coefficient is the principal function of breaking distance. You require enough friction between the road surface and the tyres to bring the car to a standstill after braking. The higher the traction coefficient the shorter the braking distance.

Which Factors Affect Stopping Distance?

Tyres

Worn tyres are unable to produce enough friction, which reduces the traction coefficient and causes them to slide over the road surface without stopping regardless of the quality of the braking system.

Over and under-inflation are key factors in wearing out tyres and reducing tyre- to-road contact.

Under-inflation causes excessive flexing of the tyre sidewall, which leads to overheating, and ultimately, casing break-up and tread separation.

Over-inflation makes the tyre more susceptible to impact, penetration, and abrasion.

The tyre compound or makeup can also change braking distance. High performance tyres typically offer better adhesion under heavy braking and will not break loose or skid as easily as general use tyres.

Braking and Suspension Systems

According to road safety advocates Arrive Alive, worn brake drums, rotors, pads, shoes, or leaky brake lines all have an adverse effect on braking. Since weight transfer has so much to do with braking, worn shock absorbers and springs will allow the weight to move to the front of the car which will add distance to stopping distance when you hit the brakes hard.

Road Conditions

Dirt and gravel roads do not offer good traction and will therefore increase braking distance. Freshly paved asphalt offers the best adhesion.

Weather Conditions

Driver Ability

Driver reaction time is a critical component in calculating and determining stopping speed. The average reaction time estimate varies between 1.5 - 2 seconds and is as high as 2.5 seconds in older drivers.

Driving experience and being familiar with your car are equally key to safe and quick braking.

How to Calculate Stopping Distance Quickly

Make a habit of calculating your stopping distance between your car and other objects around you – it can save not only money but lives too.

Braking distance in good, dry road conditions with good tyres and brakes is calculated 0.4 meters at 10 km/h.

A quick way to calculate your stopping distance is by the length of your car and your travelling speed (km/h). Typical stopping distances for an average family sedan are:

30 km/h = 3 x

50 km/h = 6 x

60 km/h = 9 x

100 km/h = 18 x

Basic Rules About Tyre Safety

The critical importance of tyres in braking efficiency underscores the need to keep them at their optimum level of functionality. This can be done by observing the following basic tyre safety rules:

Only use tyre sizes recommended by the manufacturer.

Ensure that the tyre brand, size and tread pattern are the same on each axle.

Keep tyres, including the spare, at the recommended inflation by checking them at least once a week and at low ambient temperature first thing in the morning.

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